Recently a homogeneous liquid-phase ethylene oxide (EO) process with nearly total EO selectivity, catalyzed by methyltrioxorhenium with H2O2 as an oxidant, was reported. Fundamental mass transfer and kinetic studies of this reaction are reported in the present work. Volumetric expansion studies revealed that the liquid reaction phase (methanol + H2O2/H2O) is expanded by up to 12% by compressed ethylene in the 20-40 degrees C range and up to 50 bars. This represents an increase in ethylene solubility by approximately one-order of magnitude, attributed to the unique exploitation of near-critical ethylene (P-c = 50.76 bar; T-c = 9.5 degrees C). Interphase mass-transfer coefficients for ethylene dissolution into the liquid phase were obtained experimentally. Operating at conditions that enhanced the ethylene solubility and eliminated interphase mass-transfer limitations maximized the EO productivity (1.61-4.97 g EO/h/g cat), rendering it comparable to the conventional process. Intrinsic kinetic parameters, estimated from fixed-time semibatch reactor studies, disclosed the moderate activation energy (57 +/- 2 kJ/mol). (C) 2012 American Institute of Chemical Engineers AIChE J, 59: 180-187, 2013